In oil and gas well drillings, drilling fluids (also referred to as drilling muds) generally play an important role in keeping the drilling well stable, and enable drilling to the desired depth with safety and rapidity.
Accordingly, improper controlling of mud for the well formation develops into sources of accidents such as breaks, stuck pipes, blow outs, etc.
In general, the drilling fluids widely used up to this time are mainly composed of water-bentonite suspension and are fluids that are regulated so as to accord with the well formation by addition of dispersants, viscosifiers, colloidal protectors, water loss controllers, soluble salts, surfactants, lubricants, weighting materials, and other drilling mud additives of various kinds in accordance with the particular intended use.
It is noted that sodium carboxymethyl cellulose (CMC) tends to cover clay particles with its long chain molecules, and pluggs porosities of the mud wall, and thus has extremely excellent properties as a colloidal protector and water loss controller, and accordingly has been widely employed for the purpose of preventing the breaking of formation walls.
Accordingly, the drilling fluids with the purpose described above as the principal object are desirably fluids that are mainly composed of water, bentonite, and CMC, and further that the various mud materials are added in accordance with the particular intended use. Typical fluids are shown in Table 1.
TABLE 1 ______________________________________ Examples of Drilling Fluids (parts by weight) Fresh Water- Seawater- Mud Material Bentonite Mud Bentonite Mud ______________________________________ fresh water 100 -- seawater -- 100 bentonite 1-10 8-15 CMC 0.1-1.0 0.5-1.5 asbestos -- 1-2 sodium hydroxide -- 0.2-0.5 sodium carbonate -- 0.5-1 ______________________________________
However, when the temperature in a well becomes higher than about 80.degree. C., the above drilling fluids are extremely unstable and have the drawback of increasing water loss. Therefore, it has been found that when such drilling fluids are used under high temperature conditions, due to the lowering of their efficacies as described above, it becomes difficult to drill the formation, and furthermore that the break of the formation wall occurs in some cases.